ASME II Part D - Allowable Stress Values - Novice Question

[Mark1arm]

Hi There,

I'm very new to pressure vessel engineering especially ASME.

At the moment I am trying to understand where things come from and how they are applied, so please excuse me if me question sounds very amateurish .

With regards to the allowable stress values stated in ASME II Part D, I'm trying to get my head around where the allowable stress values come from. For instance, I have been informed that a safety factor of 3.5 is now in affect in determining the allowable stress values of materials properties. Can someone please tell me where this is stated and what figures are used in conjunction to calculate the stress values.

mANY tHANKS!

 

[XL83NL]

Good question, but I suggest next time you first do a search (either this website search function or through Google).
Numerous topics on this subject can be found @ eng-tips.com. To help you out, here are a few I could find that discuss this subject in some way;

http://www.eng-tips.com/viewthread.cfm?qid=370904

http://www.eng-tips.com/viewthread.cfm?qid=358007

http://www.eng-tips.com/viewthread.cfm?qid=233127

A more direct answer; ASME II-D mandatory appendix 1

 

[Mark1arm]

Many thanks for the reply! Much appreciated! I will certainly do this in the future.

However I'm still really struggling trying to interpret this. Basically I have some calculation data (done by an ex-employee) here which is for 304L stainless steel. He states that the allowable stress value is 16700psi. I'm just trying to figure out where or how this has been achieved. As I previously mentioned I have hardly any experience which code work especially ASME and no one else here does either.

We have a customer who is wanting an old design re-built but with the allowable stress values used from 2001 Edition.

I would be more than grateful if someone where able to guide me or compose a short tutorial type reply.

Kind Regards,

Mark.

 

[XL83NL]

ASME II-D lists stress allowables for several types of material, based on a specific material form (like forging, bar, plate, or pipe).
Each form has a different ASTM A or B spec (in for Code purposes, thus becomes ASME SA or SB), listed in ASME II-D.

E.g., for 316 forging to ASTM A182 F316, the allowables may be different than for 316 plate (to ASTM A240) at a certain temperature.
This is due to the manufacturing process, heat treatment, and controlling thickness.
Hence it's of key importance to determine the relevant product form under consideration, and, from the relevant ASTM/ASME spec, derive the stress allowables.

Furthermore, allowables for a specific grade in a specific form may change over time (search for topic on why this - I think I even hyperlinked on such thread).
One reason is due to advances/improvements in and better control manufacturing techniques.
So allowables in the respective ASME II-D edition that was used for your '2001 design' may have different allowables to the latest ASME II-D (being 2013 edt.).

Second, since a new vessel is for new constrution, even though it's a copy from an old, I'd suggest you design it to the latest Code rules, thus lastest edt of ASME II-D.

 

[Mark1arm]

Thanks, makes sense!

I'm just at a loss as to where the 3.5 safety factor comes into the whole process. For instance for 304 stainless it is 3.5/515 = 147.1.... OK that's great but what do I do with that figure. So then there is the room temperature figure overleaf that is 138mpa at room temperature :. 138 x 145 = 2000psi which is what I use to input to determine the thickness of the material.

Where does all this tie in more importantly what should I be doing with this 3.5 safety factor.

once again sorry, I'm trying to learn.

Many Thanks.

 

[TomBarsh]

Mark1arm, refer to ASME Section II Part D, Mandatory Appendix 1 (the others are valuable as well) for discussion of how the tabulated allowable stress values are determined and how the 3.5 factor is used. Note that in ASME the allowable stresses used for design must be taken from the tables, not calculated at will. Also, the tabulated values are rounded for rationality; this is also described in the appendices, somewhere, I forget where.

 

[Mark1arm]

Brilliant, thanks Tom, that also makes sense. I've been over thinking the process. Therefore I'm assuming the factor of safety is somewhat built into to the data that has been tabulated.

Many Thanks!

 

[XL83NL]

Mark1arm, refer to ASME Section II Part D, Mandatory Appendix 1 (the others are valuable as well) for discussion of ...

see my first post

 

[TGS4]

When you read the Appendices in Part D (note that there are multiple Appendices because II-D covers more than just Section VIII, Division 1), you will note that there is not one single design margin (the term factor of safety is inappropriate for these discussions, BTW). Rather, there is a collection of different margins against different criteria, and the one that results in the lowest allowable stress is the governing one.

For example, there is a design margin of 1.5 on yield (both room temperature and design temperature). And there are various factors once you get into the creep regime.

 

[Mark1arm]

Fantastic. Very helpful guys! I appreciate your input on this. As I said I'm trying to pick up these codes as the main ASME guy has now left our company so I've been thrown in the deep end! I'm sure I will be back on here very soon

 

[ijzer]

See ASME IID appendix 10. Here you will find the basis of the allowable stress determination

 

[XL83NL]

See ASME IID appendix 10. Here you will find the basis of the allowable stress determination

That appendix is for tables 5A and 5B, which are the Section VIII, Division 2 Maximum Allowable Stress Values S[sub]m[/sub] for Ferrous Materials and Nonferrous Materials, respectively.

 

[ijzer]

Sorry, I ment Appendix 1, of ASME IID